Solar cells are photovoltaic (PV) devices that convert light into electrical energy. Solar cells have been developed as clean, renewable energy sources to meet growing demand. Solar cells have been implemented in a wide number of commercial markets including residential rooftops, commercial rooftops, utility-scale PV projects, building integrated PV (BIPV), building applied PV (BAPV), PV in electronic devices, PV in clothing, etc. Currently, crystalline silicon solar cells (both single crystal and polycrystalline) are the dominant technologies in the market. Crystalline silicon solar cells must use a thick substrate (>100 um) of silicon to absorb the sunlight since it has an indirect band gap. Also, the absorption coefficient is low for crystalline silicon because of the indirect band gap. The use of a thick substrate also means that the crystalline silicon solar cells must use high quality material to provide long carrier lifetimes to allow the carriers to diffuse to the contacts. Therefore, crystalline silicon solar cell technologies lead to increased costs. Thin film solar photovoltaic (TFPV) devices based on amorphous silicon (a-Si), CIGS, CdTe, CZTS, etc. provide an opportunity to increase the material utilization since only thin films (<10 um) are generally required. The thin film Si-based solar cells may be formed from amorphous, nanocrystalline, micro-crystalline, or mono-crystalline materials. TFPV devices may have a monolithic configuration (i.e. they are comprised of a single light conversion device) or they may have a tandem configuration wherein multiple TFPV devices are used to increase the absorption efficiency within different wavelength regions of the incident light spectrum.
TFPV devices provide an opportunity to reduce energy payback time, and reduce water usage for solar panel manufacturing. CdTe and CZTS films have band gaps of about 1.5 eV and therefore, are efficient absorbers for wavelengths shorter than about 800 nm. The absorption coefficient for CdTe is about 105/cm and the absorption coefficient for CZTS is about 104/cm. CIGS films have bandgaps in the range of 1.0 eV (CIS) to 1.65 eV (CGS) and are also efficient absorbers across the entire visible spectrum. The absorption coefficient for CIGS is also about 105/cm. Additionally, thin film devices can be fabricated at the module level, thus further decreasing the manufacturing costs. Furthermore, thin film devices may be fabricated on inexpensive substrates such as glass, plastics, and thin sheets of metal. Among the thin film solar technologies, CIGS has demonstrated the best lab cell efficiency (close to 20%) and the best large area module efficiency (>15%).
A general class of PV absorber films of special interest is formed as multinary compounds from Groups IB-IIIA-VIA of the periodic table. Group IB includes Cu, Ag, and Au. Group IIA includes B, Al, Ga, In, and TI. Group VIA includes O, S, Se, Te, and Po. Additionally, the IB-IIIA-VIA materials can be doped with dopants from Groups VIII, IIB, IVA, VA, and VIIA of the periodic table. Group VII includes Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt. Group IIB includes Zn, Cd, and Hg. Group IVA includes C, Si, Ge, Sn, and Pb. Group VA includes N, P, As, Sb, and Bi. Group VIIA includes F, CI, Br, I, and At. Other potential absorber materials of interest include cuprous oxide, iron sulfide, etc.
TFPV devices can be fabricated at the cell level or the panel level, thus further decreasing the manufacturing costs. As used herein, the cell level is understood to mean an individual unit that can be combined with other units to form a module. As used herein, the panel level is understood to mean a large monolithic TFPV structure that is not composed of smaller units. Generally, the panels are similar in size to the aforementioned modules. For economy of language, the phrase “TFPV device” will be understood to refer to either a solar cell or a panel without distinction. Furthermore, TFPV devices may be fabricated on inexpensive substrates such as glass, plastics, and thin sheets of metal. Examples of suitable substrates comprise float glass, low-iron glass, borosilicate glass, flexible glass, specialty glass for high temperature processing, stainless steel, carbon steel, aluminum, copper, polyimide, plastics, etc. Furthermore, the substrates may be processed in many configurations such as single substrate processing, multiple substrate batch processing, inline continuous processing, roll-to-roll processing, etc.
The increasing demand for environmentally friendly, sustainable and renewable energy sources is driving the development of large area, thin film photovoltaic (TFPV) devices. With a long-term goal of providing a significant percentage of global energy demand, there is a concomitant need for Earth-abundant, high conversion efficiency materials for use in photovoltaic devices. A number of Earth abundant direct-bandgap semiconductor materials now seem to show evidence of the potential for both high efficiency and low cost in Very Large Scale (VLS) production (e.g. greater than 100 gigawatt (GW)), yet relatively little attention has been devoted to their development and characterization remains difficult because of the complexity of the materials systems involved.
Among the TFPV technologies, CIGS and CdTe are the two that have reached volume production with greater than 10% stabilized module efficiencies. Solar cell production volume must increase tremendously in the coming decades to meet sharply growing energy needs. However, the supply of indium (In), gallium (Ga) and tellurium (Te) may inhibit annual production of CIGS and CdTe solar panels. Moreover, price increases and supply constraints in In and Ga could result from the aggregate demand for these materials used in flat panel displays (FPD) and light-emitting diodes (LED) along with CIGS TFPV. Also, there are concerns about the toxicity of Cd throughout the lifecycle of the CdTe TFPV solar modules. Efforts to develop devices that leverage manufacturing and R&D infrastructure related to these TFPV technologies but using more widely available and more environmentally friendly raw materials should be considered a top priority for research. The knowledge and infrastructure developed around CdTe and CIGS TFPV technologies can be leveraged to allow faster adoption of new TFPV materials systems.
The immaturity of TFPV devices exploiting Earth abundant materials represents a daunting challenge in terms of the time-to-commercialization. That same immaturity also suggests an enticing opportunity for breakthrough discoveries. A quaternary system such as CIGS or CZTS requires management of multiple kinetic pathways, thermodynamic phase equilibrium considerations, defect chemistries, and interfacial control. The vast phase-space to be managed includes process parameters, source material choices, compositions, and overall integration schemes. Traditional R&D methods are ill-equipped to address such complexity, and the traditionally slow pace of R&D could limit any new material from reaching industrial relevance when having to compete with the incrementally improving performance of already established TFPV fabrication lines.
However, due to the complexity of the material, cell structure and manufacturing process, both the fundamental scientific understanding and large scale manufacturability are yet to be improved for CIGS and CZTS TFPV devices. As the photovoltaic industry pushes to achieve grid parity, much faster and broader investigation is needed to explore the material, device, and process windows for higher efficiency and a lower cost of manufacturing process. Efficient methods for forming different types of CIGS and CZTS TFPV devices that can be evaluated are necessary.
The efficiency of TFPV devices depends on many properties of the absorber layer and the buffer layer such as crystallinity, grain size, composition uniformity, density, defect concentration, doping level, surface roughness, etc. Many of these properties may be improved by annealing the layers at high temperatures (i.e. about 1000 C). However, this is above the melting point of soda lime glass (600 C), a common substrate for thin film solar cells, as well as other potential substrates such as plastic sheets.
The manufacture of TFPV devices entails the integration and sequencing of many unit processing steps. As an example, TFPV manufacturing typically includes a series of processing steps such as cleaning, surface preparation, deposition, patterning, etching, thermal annealing, and other related unit processing steps. The precise sequencing and integration of the unit processing steps enables the formation of functional devices meeting desired performance metrics such as efficiency, power production, and reliability.
As part of the discovery, optimization and qualification of each unit process, it is desirable to be able to i) test different materials, ii) test different processing conditions within each unit process module, iii) test different sequencing and integration of processing modules within an integrated processing tool, iv) test different sequencing of processing tools in executing different process sequence integration flows, and combinations thereof in the manufacture of devices such as TFPV devices. In particular, there is a need to be able to test i) more than one material, ii) more than one processing condition, iii) more than one sequence of processing conditions, iv) more than one process sequence integration flow, and combinations thereof, collectively known as “combinatorial process sequence integration”, on a single monolithic substrate without the need of consuming the equivalent number of monolithic substrates per material(s), processing condition(s), sequence(s) of processing conditions, sequence(s) of processes, and combinations thereof. This can greatly improve both the speed and reduce the costs associated with the discovery, implementation, optimization, and qualification of material(s), process(es), and process integration sequence(s) required for manufacturing.
Systems and methods for High Productivity Combinatorial (HPC) processing are described in U.S. Pat. No. 7,544,574 filed on Feb. 10, 2006, U.S. Pat. No. 7,824,935 filed on Jul. 2, 2008, U.S. Pat. No. 7,871,928 filed on May 4, 2009, U.S. Pat. No. 7,902,063 filed on Feb. 10, 2006, and U.S. Pat. No. 7,947,531 filed on Aug. 28, 2009 which are all herein incorporated by reference. Systems and methods for HPC processing are further described in U.S. patent application Ser. No. 11/352,077 filed on Feb. 10, 2006, claiming priority from Oct. 15, 2005, U.S. patent application Ser. No. 11/419,174 filed on May 18, 2006, claiming priority from Oct. 15, 2005, U.S. patent application Ser. No. 11/674,132 filed on Feb. 12, 2007, claiming priority from Oct. 15, 2005, and U.S. patent application Ser. No. 11/674,137 filed on Feb. 12, 2007, claiming priority from Oct. 15, 2005 which are all herein incorporated by reference.
HPC processing techniques have been successfully adapted to wet chemical processing such as etching and cleaning. HPC processing techniques have also been successfully adapted to deposition processes such as physical vapor deposition (PVD), atomic layer deposition (ALD), and chemical vapor deposition (CVD). However, HPC processing techniques have not been successfully adapted to the development of TFPV devices.
Traditionally, TFPV devices have been manufactured with two glass plates, one on either side of the device to provide support and encapsulation. One version of TFPV device is known as a “superstrate” device. In a superstrate configuration, one glass plate is used as both a support and as the window for illumination. Therefore, the second glass plate is not required and a more cost effective encapsulate may be used to seal the TFPV device. Typically, the cost of the supporting glass plates has been similar to the active electronic materials. Therefore, TFPV devices with a superstrate configuration present an opportunity to lower the manufacturing cost for TFPV devices.
The process flow for the manufacture of superstrate TFPV devices is different from the conventional process flow. The materials, process parameters, unit processes, process sequences, process integration, etc. need to be improved to increase the efficiency.
Therefore, there is a need to apply high productivity combinatorial techniques to the development and investigation of materials and processes for the manufacture of superstrate CdTe, CIGS, and CZTS superstrate TFPV devices.